Many computer vision systems for automatic surveillance and monitoring seek to detect and segment transitory objects that appear temporarily in the system's field of view. Examples include traffic monitoring applications that count vehicles and automatic surveillance systems for security. These systems often require different object detection and segmentation methods depending on the ambient conditions. An example of such a system is disclosed in U.S. patent application Ser. No. 08/372,924 filed Jan. 17, 1995, the disclosure of which is incorporated herein by reference.
The three primary ambient conditions that can create a need for different detection and segmentation methods are: scene illumination, for example whether it is day or night, the presence of shadows, and scene is obscured due to log, smoke or haze. For example, in a traffic monitoring system, the detection method at night may be specialized for detecting headlights and therefore may not be applicable during the daytime. Also, on bright days, objects may cast shadows that interfere with accurate object segmentations, and therefore require that an additional shadow removal method be used. Finally, on very foggy days, reliable object detection may be impossible, and therefore the monitoring system operates in a fail-safe mode.
If an automatic monitoring system is to operate autonomously over an extended period of time, it should preferably include a method for determining the ambient conditions within a scene so that the system can use the appropriate detection method in response to those conditions.
Previous work on determining scene illumination, shadow presence, or fog presence has been based on image analysis. Some deployed systems predict scene illumination and shadow presence using an internal clock, knowledge of latitude and longitude, and a precomputed calendar of sun positions. This, however, is not robust to overcast days or foggy weather.
Additionally, approaches based on raw image intensity thresholds are very unlikely to be robust.
Therefore, there is a need for a method and apparatus for determining the ambient conditions in a scene including scene illumination and the presence of shadows, fog, smoke or haze.